a) Field of the Invention
The present invention relates to position detection techniques using scattered light from edges or apexes, and more particularly to a position detection method, a position detection apparatus and an alignment mark, suitable for improving throughput of proximity exposure.
b) Description of the Related Art
As the position alignment of a wafer and a mask by using an alignment system composed of a lens system and an image processing system, a vertical detection method and an oblique detection method are known. The vertical detection method observes alignment marks along a direction vertical to the mask surface, whereas the oblique detection method observes alignment marks along a direction oblique to the mask surface.
A chromatic bifocal method is known which is used as a focussing method for the vertical detection method. With this chromatic bifocal method, a mask mark formed on a mask and a wafer mark formed on a wafer are observed with light having different wavelengths, and focussed on the same flat plane by utilizing chromatic aberration of the lens system. This method can principally set an optical resolution of a lens high, so that an absolute position detection precision can be made high.
However, since alignment marks (the mask marks and the wafer marks) are observed along the vertical direction, the observing optical system enters an exposure region. If exposure is performed in this state, the optical system intercepts exposure light. It is therefore required to retract the optical system from the exposure region when exposure is performed. The time required for the optical system to retract from the exposure region lowers throughput. Further, during the exposure, the alignment marks cannot be observed and their positions cannot be detected. This may cause a low alignment precision during the exposure.
With the oblique detection method, the optical system is disposed with its optical axis being set oblique to the mask surface so that it can be located at the position not intercepting exposure light. It is therefore unnecessary to retract the optical system during exposure, and alignment marks can be observed even during exposure. It is possible to prevent position misalignment during exposure, without lowering throughput.
With this oblique detection method, however, wafer and mask marks observed obliquely are focussed so that an absolute precision of position detection is lowered by image distortions. Furthermore, since the optical axis of illumination light is not coincident with the optical axis of observation light, it is not possible to coaxially dispose both axes. Therefore, the illumination light axis becomes easy to shift from an ideal optical axis. As the illumination light axis shifts from the ideal optical axis, images are distorted and correct position detection becomes difficult.